Microwave ovens have failed to meet its full cooking potential due to three distinct problems. First, there is the inability to generate uniform temperature distributions within bulk products, due to the finite penetration depth of the microwaves causing heavy perimeter heating with an accompanying electrical quietness in the centre of the product. Second, there is an inability to brown and crisp items in a similar way to conventional ovens caused by the absence of surface power dissipation created by a) the ability of microwaves to penetrate the bulk and b) the low ambient air temperature generally found in a microwave oven. Third, there is an inability to control the relative heating rates of materials as a result of the dielectric properties of the materials becoming the dominant factor in the heating rates, since different materials with different dielectric properties will heat at different rates in the microwave oven and therefore control over multi-component meals becomes lost.
A good deal of work has gone into creating materials or utensils that permit foods to be cooked in a microwave oven and to give outcomes that are similar to a conventional oven's performance. The most popular device being used is a microwave susceptor material. Microwave susceptors are quite effective in generating surface heat and so can contribute significantly to crisping of surfaces. However microwave susceptors do not have any ability to modify the field environment and so their ability to redistribute power within the microwave oven is quite limited.
Other solutions propose the use of metallic structures to redistribute power or to change the nature of the propagation of the microwave power. The basic tenant of how such structures would work is that they should be able to carry large microwave currents within themselves. These structures typically consist of three different features.
First, large continuous sheets of metal may be used to act as a shield protecting the adjacent food materials from exposure to microwaves. Second, resonant elements can be used to enhance bulk heating and to equalize voltages over a fairly large area. In addition, undersized elements that would otherwise be resonant at much higher frequencies can be used to promote evanescent propagation into materials causing a loss of surface power dissipation. Third, metallic elements can be used as transmission components to permit either redistribution of power or the enhanced excitation of localized susceptors.
The effectiveness of metallic structures to change the power distribution in microwaves is based upon the structure's ability to carry microwave currents. In most applications the components that are carrying the currents would be in fairly close proximity to the food, so the food would act as a load in two manners. First, the food would act as a microwave absorbing load, which would dampen the voltages and currents on the various elements. Second, the food would act as a thermal load, acting as a large heatsink ensuring that the substrate or the metallic elements do not overheat.
A serious problem exists for consumer applications. It is impossible to control abuses of the microwave packaging. Examples of such abuses include packages that are incorrectly assembled either at the packaging manufacturer or the food processor, or indeed within the domestic environment. Packages are often damaged during unpacking and display. The cartons in which the microwave packages are shipped are often cut with a blade to open the carton which usually results in several of the microwave packages being cut in the process. The metallic elements designed for intercepting microwave current will generated high voltages across the cut creating a fire hazard.
Consumers may remove all or part of the food load and attempt to cook without the designed food load. The removal of the food load may be as simple as eating half the product and expecting to be able to reheat the other half in the supplied packaging. For many types of metallic elements proposed in the prior art, this removal of the food or any abuse conditions can represent a significant threat to the consumers safety. Removing the food load removes both the electrical and thermal load on the metallic elements. The result may often be that a free standing element when exposed to microwave oven voltages, which for a small load can be in the order of ten to twelve thousand volts per meter for a characteristic microwave oven rated at 900 watts, can stimulate arcing and subsequent fire or heat the substrates to the point where they spontaneously combust. The result is clearly a consumer threat that can either damage the microwave oven or worse, cause personal injury or further damage to components outside the microwave oven if the fire is not contained in a proper manner.